Optimizing saugeye sampling protocol

W 2800.7 F532r/w No.9
1996/97 c.2
[AL REPORT
RESEARCH AND SURVEYS
$ * > %
ORjC
FEDERAL AID GRANT NO. F-50-R
FISH RESEARCH FOR OKLAHOMA WATERS
PROJECT NO. 9
OPTIMIZING SAUGEYE SAMPLING PROTOCOL
MARCH 1, 1996 through FEBRUARY 28, 1997
FINAL REPORT
State: Oklahoma Grant Number: F-50-R
Grant Title: Fish Research for Oklahoma Waters
Project Number: 9
Project Title: Optimizing saugeye sampling protocol.
Contract Period: From: March 1. 1996 To: February 28. 1997
ABSTRACT
Monthly electrofishing samples for saugeye (walleye x sauger hybrids; Stizostedion vitreum vitreum x S.
canadense) were collected on three lakes during spring and fall, 1996. Sampling was stratified by day and night and
habitat type. Catch per hour (CPUE) was calculated for four size classes and compared for each sampling strata.
Precision of the estimates was calculated and sampling recommendations made.
Differences in seasonal catch rates were inconsistent between lakes and among size classes. CPUE's of night
samples were higher for all lakes for the "small" and "intermediate" size classes. However, no clear diel pattern in
catch rates of "large" saugeye were observed. Habitat type had little effect on sampling efficiency. Precision of most
samples was poor. Ten hours of electrofishing would be needed to obtain estimates ±75% of the mean.
Sampling recommendations included collecting data on the "small" and "intermediate" size classes using fall
night electrofishing. Data on "large" saugeye could be collected during the fall night samples and also during spring,
day time electrofishing sampling targeting largemouth bass. In order to collect statistically reliable data, sampling effort
needs to be increased 7-fold over existing sampling protocols.
I. Problem or Need:
With increasing demands placed on the time of fisheries management staff and ever shrinking budgets,
increased sampling efficiency is imperative to effective management programs. Further complicating the balance
between available time and funds and collecting quality information, is the increased awareness that collecting
statistically reliable data is often dependent on designing species-specific sampling programs (Parrish et al. 1995).
The Oklahoma Department of Wildlife Conservation (ODWC) has been stocking saugeye (walleye-sauger
hybrids; Stizostedion vitreum vitreum x S. canadense) since 1985. Saugeye stockings have become an important part
of Oklahoma's fisheries management program (saugeye were stocked in 18 lakes in 1996). The ODWC has developed
specific stocking criteria and objectives for the saugeye stocking program (Gilliland and Boxrucker 1995). One of these
objectives has been to control slow-growing and/or stunted crappie populations. However, for saugeye to be effective
predators on crappie, they must be approximately 50 cm TL (Horton and Gilliland 1991). Current saugeye sampling
procedures (night electrofishing and gill netting) do not adequately sample adult saugeye populations (ODWC survey
data; F-44-R, Project 5). As a result, it has not been possible to set realistic target catch rates for adult saugeye to
provide effective control of overcrowded crappie populations. A statewide 18-inch length limit on saugeye is also in
effect. Without adequate sampling techniques in place, it is difficult to reliably assess the effect the regulation is having
on adult saugeye densities.
ODWC staff routinely collects spring daytime electrofishing samples for largemouth bass Micropterus salmoides.
These samples are concentrated in cove-type habitat. Preliminary data from Thunderbird Reservoir indicate that catch
rates of saugeye from samples collected off points and main-lake shoreline are higher than those from cove samples
(ODWC, unpublished data). If day time sampling in habitat typically electrofished for bass would also be conducive to
collecting quality data on saugeye, sampling efforts for these two species could be combined and overall sampling
efficiency would be improved.
Sampling was conducted on three reservoirs; Holdenville, Jean Neustadt, and Thunderbird. These reservoirs
were chosen for study largely based on past stocking history; saugeye have been stocked long enough for adult
populations to develop. The objective of this study was to determine the differences in saugeye electrofishing catch rates
and associated variability of the samples for each of four size groups by 1) month and season; 2) time of day; and 3)
habitat type.
III. Project Objective:
To determine the differences in saugeye catch rate and associated variability along with length distribution of
electrofishing samples by month, time of day, and habitat type on three reservoirs as a method to improve sampling
efficiency.
IV. Approach
Study Sites
Thunderbird Reservoir was impounded in 1965 as the municipal water supply for several central-Oklahoma
communities. It covers 2,448 ha and has a shoreline development ratio of 7.9, mean depth of 6 m, maximum depth
of 21 m, and a water exchange rate of 0.57. The lake is moderately turbid (mid-summer secchi disk readings average
approximately 60 cm). Thunderbird was first stocked with saugeye in 1985 and has since received annual stockings.
Holdenville Lake covers 223 ha and was constructed in 1932 by the City of Holdenville as a water supply.
The lake has a mean depth of 6 m and a maximum depth of 16 m, a shoreline development ratio of 3.3, a water
exchange rate of 0.36, and a secchi disk reading of 180 cm. Saugeye have been stocked annually since 1988, with the
exception of 1992.
Jean Neustadt was impounded in 1968 and covers 187 ha. It has a mean depth of 3 m and a maximum depth
of 14 m, a shoreline development ratio of 3.3, a water exchange rate of 0.76, and a secchi disk reading 76 cm. With
the exception of 1992, saugeye have been stocked annually since 1989.
Electrofishing Procedures
Electrofishing samples were collected on two dates on each lake during March, April, May, October, and
November, 1996. Samples were collected at fixed sites, with two-person crews (one dipper/one boat driver) using
pulsed DC current (60 pulses/sec; 8-10 amps). Electrofishing sites were stratified by habitat; 1) points and main-lake
shoreline, hereafter referred to as "saugeye" habitat; and 2) coves, hereafter referred to as "bass" habitat. Six 15-
minute units of effort were collected in each habitat type during daylight on each date. Six units of effort also were
collected after dark in "saugeye" habitat on each date for a total of 18 units of effort. The day and night samples in
"saugeye" habitat were collected at the same sites. Data from both sampling dates for each month were pooled (36 units
of effort per lake per month).
Saugeye were the only fish collected during sampling. All fish were measured to the nearest mm and weighed
to the nearest g and with one of four size categories used for analysis; 1) <_ 310 mm (age 0; hereafter referred to as
"small"); 2) <. 400 mm (age 0 and yearling; hereafter referred to as "intermediate"); 3) _> 457 mm (statewide
minimum length limit; hereafter referred to as "large"); and 4) all size classes combined. Spring, 1996 and fall, 1996
samples collected different year classes, particularly in the "small" and "intermediate" size classes. However, stocking
rates were the same for all lakes (50/ha) and assuming relatively similar survival of stocked fish between years, I felt
that this would not compromise the study objectives.
Catch rates (CPUE) were expressed as number of fish per hour of electrofishing. Electrofishing catch rates
for each lake were compared by month and season, day and night, and habitat type for each size group. Catch data
from all lakes were not pooled due to differences in population abundance. CPUE data were log-transformed
[loge(CPUE-f 1)1 to normalize data and stabilize variances. Standard t-tests were used to determine differences in CPUE
by season and habitat type for each lake and size class. Paired t-tests were used to test for differences between day and
night samples collected from "saugeye" habitat. Ryan's multiple comparison test was used to determine differences in
CPUE by month. Statistical significance was assessed at P=0.05.
Sampling precision was measured by determining the coefficient of variation of the mean (CVx=S.E.x1). A
target level of precision was set at CV*=0.125. This value corresponds to the x+0.25x and coincides with standards
established for "management studies " by Robson and Regier (1964). Rearranging the above equation, inserting the
desired level of precision, and solving for N (number of samples) yields the equation:
N=0.125-2x-2s2. (equation 1)
Standard equations for estimating sampling size assume that the data are normally distributed and that the sample mean
and variance are uncorrelated. A mean-variance relationship for this study was calculated from all sampling strata, lakes,
and dates combined by linear regression of loges2 on loge*, yielding the equation:
loges2=1.61+1.221ogex. (equation 2)
The regression equation relating s2 and x was back-transformed to a linear scale and corrected for transformation bias
by adding the mean square error of the regression (MSE)/2. The mean-variance relationship for all samples collected
then becomes:
s2 =exp[(MSE/2) + 1.61 + 1.22x)] (equation 3)
=exp[(0.45/2)+1.61 + 1.22x], (equation 4)
=6.26x '22. (equation 5)
These results were substituted into equation 1 and used to compute sample size requirements:
N=6.26x122x20.125-2 (equation 6)
N=6.26x0780.125-2. (equation 7)
V. Results
Differences in seasonal catch rates were not consistent among lakes nor size classes (Table 1). CPUE of
"small" saugeye was higher in the fall from Holdenville, whereas no seasonal differences in CPUE of the "small" size
class were observed in the Jean Neustadt and Thunderbird data (Table 1). CPUE of the "intermediate" size class was
higher in the fall from Jean Neustadt and Thunderbird; no seasonal difference was detected from Holdenville (Table
1). CPUE of "large" saugeye was higher in the spring from Holdenville and Jean Neustadt; however, higher CPUE's
were found in the fall samples from Thunderbird (Table 1). Catch rates for all size classes combined did not differ by
season from Holdenville and Jean Neustadt, but were higher at Thunderbird in the fall (Table 1).
Analysis of the monthly electrofishing data did not provide additional insight into temporal differences in
CPUE. Therefore, monthly comparisons will not be discussed further.
Precision of most samples was poor. Precision of the Thunderbird data was higher than that for Holdenville
and Jean Neustadt (Table 1). However, a minimum of 5 hours of electrofishing was required to obtain a CVx=0.125
in fall sampling with size classes, day time, and habitats combined (Table 1). Stratifying the data by size class generally
made sampling requirements unrealistic. Ten hours of electrofishing would meet the specified target of precision in
only three of the 24 sampling strata depicted in Table 1.
Diurnal differences in CPUE were more consistent. CPUE's of night samples were higher for all lakes for
the "small" and "intermediate" size classes (Table 2). No diurnal differences in catch rates of "large" saugeye were
found in either season at Holdenville nor in the spring from Thunderbird (Table 2). CPUE of the "large" size class was
higher at night in the spring and during the day in the fall from Jean Neustadt (Table 2). Fall CPUE of the large"
size class was higher during the day at Thunderbird. Catch rates were higher at night for size classes combined tor all
lakes and seasons with the exception of the fall samples from Thunderbird which did not exhibit any diurnal differences
in catch rates (Table 2).
Precision of the night electrofishing samples was higher than the paired samples collected during the day (Table
2). The exceptions to this relationship were at Thunderbird for "large" saugeye and for all size classes combined in
the fall (Table 2). Ten hours of electrofishing (40 units of effort) at night would be required to ensure obtaining a
mean with a 75 % confidence interval based on data from the three lakes in this study (Table 2). By stratifying the data
by day and night, precision was not sacrificed in many cases when the data were broken out by size classes. Eleven
hours of electrofishing would meet the target of precision in 19 of the 48 sampling strata in Table 2. If only night
samples were considered, 15 of 24 strata met the target of precision with 11 hours of electrofishing.
Habitat type had little effect on day time electrofishing sampling efficiency. CPUE was higher in "saugeye"
habitat for the "intermediate" size class in spring at Holdenville and in the fall at Jean Neustadt (Table 3). CPUE of
"large" saugeye was higher in the fall at Thunderbird (Table 3). Catch rates for all size classes combined in "saugeye"
habitat were higher in the spring at Holdenville and in the fall at Jean Neustadt and Thunderbird (Table 3). All other
comparisons of habitat type by size class for each season and lake were nonsignificant. Precision of the data collected
in "saugeye" habitat was higher than the respective data collected in "bass" habitat in 17 of the 24 habitat comparisons
in Table 3. However, stratifying the data by habitat type did little to reach realistic sample size requirements.
VI. Discussion:
This study provided no clear evidence indicating that efficiency would be enhanced by limiting data collection
to a single season. Seasonal differences in catch rates and precision were inconsistent among lakes and size classes.
This is in contrast to the findings of Johnson et al. (1988) who reported higher catch rates of age-1 and older saugeye
from night electrofishing samples in spring than in fall from Pleasant Hill Reservoir, Ohio. Stratifying sampling by
habitat type during daytime electrofishing also did little to improve efficiency.
7
Sampling at night clearly improved efficiency, particularly for the "small" and "intermediate" size classes.
This is consistent with sampling recommendations for collecting age-0 and yearling walleye (McWilliams and Larscheid
1992; Serns 1982). No clear evidence was found indicating that night sampling efficiency for "small" and
"intermediate" saugeye could be improved by stratifying the sampling by season. Differences in CPUE were
inconsistent between spring and fall samples; however, precision of the fall night electrofishing samples was typically
higher than the respective samples collected in spring. In addition, stocking criteria used by ODWC's management staff
(Gilliland and Boxrucker 1995) require data be provided from previous year's stocking to receive subsequent stockings.
Therefore, fall collections of age-0 saugeye fit better into existing management protocol.
Improved capture efficiency was not evident for the "large" size class at night. No differences in CPUE were
observed in the seasonal day-night comparisons from Holdenville. However, catch rates of "large" saugeye from
Holdenville were low throughout the study indicating low population density . This may make drawing any conclusions
relative to capture efficiency of "large" individuals from the Holdenville data suspect. CPUE of "large" saugeye was
higher at night for spring samples and higher during the day in the fall from Jean Neustadt. No diel differences were
seen in the spring data from Thunderbird; however, day samples in the fall had a higher CPUE. Precise data on
"large" saugeye are needed to enhance ODWC's crappie management efforts. Saugeye are used as a tool to reduce
density of overcrowded crappie populations (Gilliland and Boxrucker 1995). It would be useful to develop correlations
between CPUE of "large" saugeye and improvements in crappie size structure and/or growth rates. However, no single
sampling strategy improving capture efficiency was evident from this study.
ODWC's management staff typically expends six units of effort or less per lake to collect data evaluating
saugeye stocking success (catch/h of age-0 saugeye in fall night electrofishing). This amount of effort appears
insufficient to provide the precision needed to meet standards suggested in the literature. Effort needs to be increased
7-fold to provide estimates +.25% of the mean. However, given current time and personnel restraints, an increase in
sampling effort of this magnitude may not be practical. Given a catch rate of 30/h, a resonable estimate for saugeye
<_ 400 mm in fall night electrofishing (Table 2), a 50% change in the mean could be detected with 7 units of effort.
This amount of effort is similar to what is currently being spent. If the objective of the sampling is to evaluate
abundance of "large" saugeye, 28 samples would be needed to detect a 50% change in the mean, given a CPUE of 5
8
is assumed. Decreasing the precision of the abundance estimates dilutes our ability to detect cause and effect
relationships. As a result, our ability to refine stocking rates, detect environmental and biological influences on
survival, and correlate abundance of "large" saugeye with improvements in crappie population structure would be
compromised. CPUE is also one criteria used to prioritize annual saugeye stocking requests (Gilliland and Boxrucker
1995). The lack of precision of historical and future estimates makes objective among lake comparisons of CPUE for
prioritization purposes difficult.
VII. Recommendations:
1. Fall night electrofishing samples should be used to evaluate abundance of age-0 and yearling saugeye
populations. However, effort needs to be increased substantially (10 hours/lake) to provide estimates of
sufficient reliability on which to base management decisions. The amount of electrofishing effort currently
being spent is sufficient to detect a 50% change in abundance.
2. Data on "large" saugeye should be collected during the fall night electrofishing sampling. Since habitat
type had little influence on sampling efficiency, data on "large" saugeye should also be collected during routine
largemouth bass sampling efforts (spring, day time). Continued critical analysis of these data are needed.
Hopefully as the precision of the data is improved, sampling protocol for "large" saugeye will be refined.
3. The stocking criteria currently being used (Gilliland and Boxrucker 1995) needs continued updating as
refinements in sampling procedures are made.
III. Prepared by:
Jeff Boxrucker
Biologist HI
IV. Date: April 1, 1997
V. Approved by:
Dr. Harold Namminga
Federal Aid Research Coordinator
IX. Literature Cited
Gilliland, E.R. and J. Boxrucker. 1995. Species specific guidelines for stocking reservoirs in Oklahoma. Pages 144-
155 in H.L. Schramm and R.G. Piper, editors. Uses and effects of cultured fishes in aquatic ecosystems.
American Fisheries Society Symposium 15, Bethesda, Maryland.
Horton, R.A. and E.R. Gilliland. 1991. Diet overlap between saugeye and largemouth bass in Thunderbird Reservoir,
Oklahoma. Proc. Annu. Conf. Southeast. Assoc. Fish and Wildl. Agencies. 44: 98-104.
Johnson, B.L., D.L. Smith, and R.F. Carline. 1988. Habitat preferences, survival, growth, foods, and harvests of
walleyes and walleye x sauger hybrids. North Am. J. Fish. Manage. 8: 292-304.
McWilliams, R.H. and J.G. Larscheid. 1992. Assessment of walleye fry and fingerling stocking in the Okoboji Lakes,
Iowa. North Am. J. Fish. Manage. 12: 329-335.
Parrish, D.L., M.E. Mather, and R.A. Stein. 1995. Problem-solving research for management: A perspective.
Fisheries 20: 6-12.
Robson, D.S., and H.A. Regier. 1964. Sample size in Petersen mark-recapture experiments. Trans. Am. Fish. Soc.
93: 215-226.
Serns, S.L. 1982. Relationship of walleye fingerling density and electrofishing catch per effort in northern Wisconsin
lakes. North Am. J. Fish. Manage. 2: 38-44.
10
Table 1. Catch per hour (CPUE) and standard error (S.E.) of saugeye by electrofishing by size class from spring and fall samples collected from
Holdenville, Jean Neustadt, and Thunderbird Reservoirs, 1996. N=number of samples to obtain CV*=0.125. CPUE's within same column with same
letter are not significantly different (t-test; P<0.05). Differences across size classes and lakes were not tested.
HOLDENVILLE
Season
Spring
Fall
CPUE
2.07a
10.67b
:< 310
S.E.
0.49
2.35
mm
N
365
70
CPUE
6.56a
12.33a
^ 4 0 0
S.E.
0.87
2.53
mm
N
109
62
CPUE
1.41a
0.67b
> 457
S.E.
0.24
0.24
mm
N
589
1678
CPUE
9.74a
14.11a
All Sizes
S.E.
1.11
2.61
N
76
55
JEAN NEUSTADT
Spring
Fall
6.93a
7.56a
1.73
1.51
104
96
7.22a
9.89b
1.75
1.73
100
75
5.74a
1.11b
0.83
0.47
124
805
14.04a
11.28a
2.00
1.72
55
67
THUNDERBIRD
Spring
Fall
11.77a
8.56a
1.83
1.84
64
85
16.04a
23.61b
2.26
2.91
49
35
8.94a
14.44b
1.02
1.94
82
54
31.77a
45.72b
3.09
3.93
28
21
11
Table 2. Catch per hour (CPUE) and standard error (S.E.) of saugeye by electrofishing by season and size class from day and night samples collected from
Holdenville, Jean Neustadt, and Thunderbird Reservoirs, 1996. N=number of samples to obtain CV*=0.125. CPUE's within same column with same letter are
not significantly different (paired t-test; P<0.05). Differences across size classes, lakes, and seasons were not tested.
HOLDENVILLE
Season
Spring
Fall
Daytime
Day
Night
Day
Night
CPUE
0.44a
5.67b
0.00a
31.67b
<310mm
S.E.
0.27
1.27
0.00
4.71
N
3171
126
28
CPUE
5.00a
13.89b
0.50a
36.17b
.<400 mm
S.E.
1.15
1.73
0.28
4.70
N
142
56
2624
25
CPUE
1.44a
1.67a
0.50a
0.23a
>457 mm
S.E.
0.43
0.37
0.28
0.54
N
570
476
2624
1206
CPUE
8.33a
18.56b
1.50a
39.17b
All Sizes
S.E.
0.63
4.64
1.72
2.00
N
87
43
543
23
JEAN NEUSTADT
Spring
Fall
Day
Night
Day
Night
0.89a
19.33b
3.00a
19.17b
0.39
4.54
1.00
3.32
1099
42
241
42
1.00a
19.89b
6.67a
21.67b
0.40
4.56
1.88
3.74
931
41
108
38
5.89a
8.56b
2.50a
0.00b
1.35
1.83
1.20
0.00
121
85
294
7.78a
30.67b
9.17a
21.83b
1.58
4.60
2.12
3.71
93
29
80
38
THUNDERBIRD
Spring
Fall
Day
Night
Day
Night
8.33a
18.82b
3.50a
17.83b
3.08
2.79
1.39
4.67
87
43
204
45
12.22a
24.24b
18.67a
39.83b
3..72
3.53
3.90
5.72
62
35
43
23
8.89a
7.88a
27.50a
8.67b
1.40
1.76
4.56
1.01
82
92
31
84
27.67a
38.94b
58.50a
54.50a
4.39
5.21
7.79
5.75
31
24
17
18
12
Table 3. Catch per hour (CPUE) and standard error (S.E.) of saugeye by day time electrofishing by habitat type (Bass-cove habitat; Saugeye-points and main lake
shoreline habitat) and size class from samples collected from Holdenville, Jean Neustadt, and Thunderbird Reservoirs, 1996. N=number of samples to obtain
CVx=0.125. CPUE's within same column for each season with same letter are not significantly different (t-test; P<0.05). Differences across size classes, lakes,
and seasons were not tested.
Season
Spring
Fall
Habitat
Bass
Saugeye
Bass
Saugeye
CPUE
0.11a
0.44a
0.33a
0.00a
<310mm
S.E.
0.11
0.27
0.23
0.00
N
36867
3171
5114
CPUE
0.78a
5.00b
0.33a
0.50a
,<400 mm
S.E.
0.35
1.15
0.23
0.28
HOLDENVILLE
N
1333
142
5114
2624
CPUE
1.11a
1.44a
0.67a
0.50a
>457 mm
S.E.
0.43
0.37
0.28
0.54
N
805
570
1678
2624
CPUE
2.33a
8.33b
1.67a
1.50a
All Sizes
S.E.
0.74
1.72
0.59
0.63
N
319
87
476
543
JEAN NEUSTADT
Spring
Fall
Bass
Saugeye
Bass
Saugeye
0.56a
0.89a
0.50a
3.00b
0.32
0.39
0.37
1.00
2221
1099
2624
241
0.78a
1.00a
1.33a
6.67b
0.38
0.40
0.62
1.88
1333
931
632
108
2.78a
5.89a
0.83a
2.50a
0.83
1.35
0.68
1.20
262
121
1206
294
3.67a
7.78a
2.83a
9.17b
0.96
1.58
1.04
2.12
194
93
256
80
THUNDERBIRD
Spring
Fall
Bass
Saugeye
Bass
Saugeye
8.56a
8.33a
4.33a
3.53a
3.35
3.08
1.38
1.39
85
87
164
204
12.11a
12.22a
12.33a
18.67a
4.17
3.75
3.56
3.90
63
62
62
43
10.00a
8.89a
7.17a
27.50b
2.11
1.40
1.40
4.56
74
82
101
31
29.11a
27.67a
24.17a
58.50b
6.25
4.39
4.33
7.79
30
31
35
17

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W 2800.7 F532r/w No.9
1996/97 c.2
[AL REPORT
RESEARCH AND SURVEYS
$ * > %
ORjC
FEDERAL AID GRANT NO. F-50-R
FISH RESEARCH FOR OKLAHOMA WATERS
PROJECT NO. 9
OPTIMIZING SAUGEYE SAMPLING PROTOCOL
MARCH 1, 1996 through FEBRUARY 28, 1997
FINAL REPORT
State: Oklahoma Grant Number: F-50-R
Grant Title: Fish Research for Oklahoma Waters
Project Number: 9
Project Title: Optimizing saugeye sampling protocol.
Contract Period: From: March 1. 1996 To: February 28. 1997
ABSTRACT
Monthly electrofishing samples for saugeye (walleye x sauger hybrids; Stizostedion vitreum vitreum x S.
canadense) were collected on three lakes during spring and fall, 1996. Sampling was stratified by day and night and
habitat type. Catch per hour (CPUE) was calculated for four size classes and compared for each sampling strata.
Precision of the estimates was calculated and sampling recommendations made.
Differences in seasonal catch rates were inconsistent between lakes and among size classes. CPUE's of night
samples were higher for all lakes for the "small" and "intermediate" size classes. However, no clear diel pattern in
catch rates of "large" saugeye were observed. Habitat type had little effect on sampling efficiency. Precision of most
samples was poor. Ten hours of electrofishing would be needed to obtain estimates ±75% of the mean.
Sampling recommendations included collecting data on the "small" and "intermediate" size classes using fall
night electrofishing. Data on "large" saugeye could be collected during the fall night samples and also during spring,
day time electrofishing sampling targeting largemouth bass. In order to collect statistically reliable data, sampling effort
needs to be increased 7-fold over existing sampling protocols.
I. Problem or Need:
With increasing demands placed on the time of fisheries management staff and ever shrinking budgets,
increased sampling efficiency is imperative to effective management programs. Further complicating the balance
between available time and funds and collecting quality information, is the increased awareness that collecting
statistically reliable data is often dependent on designing species-specific sampling programs (Parrish et al. 1995).
The Oklahoma Department of Wildlife Conservation (ODWC) has been stocking saugeye (walleye-sauger
hybrids; Stizostedion vitreum vitreum x S. canadense) since 1985. Saugeye stockings have become an important part
of Oklahoma's fisheries management program (saugeye were stocked in 18 lakes in 1996). The ODWC has developed
specific stocking criteria and objectives for the saugeye stocking program (Gilliland and Boxrucker 1995). One of these
objectives has been to control slow-growing and/or stunted crappie populations. However, for saugeye to be effective
predators on crappie, they must be approximately 50 cm TL (Horton and Gilliland 1991). Current saugeye sampling
procedures (night electrofishing and gill netting) do not adequately sample adult saugeye populations (ODWC survey
data; F-44-R, Project 5). As a result, it has not been possible to set realistic target catch rates for adult saugeye to
provide effective control of overcrowded crappie populations. A statewide 18-inch length limit on saugeye is also in
effect. Without adequate sampling techniques in place, it is difficult to reliably assess the effect the regulation is having
on adult saugeye densities.
ODWC staff routinely collects spring daytime electrofishing samples for largemouth bass Micropterus salmoides.
These samples are concentrated in cove-type habitat. Preliminary data from Thunderbird Reservoir indicate that catch
rates of saugeye from samples collected off points and main-lake shoreline are higher than those from cove samples
(ODWC, unpublished data). If day time sampling in habitat typically electrofished for bass would also be conducive to
collecting quality data on saugeye, sampling efforts for these two species could be combined and overall sampling
efficiency would be improved.
Sampling was conducted on three reservoirs; Holdenville, Jean Neustadt, and Thunderbird. These reservoirs
were chosen for study largely based on past stocking history; saugeye have been stocked long enough for adult
populations to develop. The objective of this study was to determine the differences in saugeye electrofishing catch rates
and associated variability of the samples for each of four size groups by 1) month and season; 2) time of day; and 3)
habitat type.
III. Project Objective:
To determine the differences in saugeye catch rate and associated variability along with length distribution of
electrofishing samples by month, time of day, and habitat type on three reservoirs as a method to improve sampling
efficiency.
IV. Approach
Study Sites
Thunderbird Reservoir was impounded in 1965 as the municipal water supply for several central-Oklahoma
communities. It covers 2,448 ha and has a shoreline development ratio of 7.9, mean depth of 6 m, maximum depth
of 21 m, and a water exchange rate of 0.57. The lake is moderately turbid (mid-summer secchi disk readings average
approximately 60 cm). Thunderbird was first stocked with saugeye in 1985 and has since received annual stockings.
Holdenville Lake covers 223 ha and was constructed in 1932 by the City of Holdenville as a water supply.
The lake has a mean depth of 6 m and a maximum depth of 16 m, a shoreline development ratio of 3.3, a water
exchange rate of 0.36, and a secchi disk reading of 180 cm. Saugeye have been stocked annually since 1988, with the
exception of 1992.
Jean Neustadt was impounded in 1968 and covers 187 ha. It has a mean depth of 3 m and a maximum depth
of 14 m, a shoreline development ratio of 3.3, a water exchange rate of 0.76, and a secchi disk reading 76 cm. With
the exception of 1992, saugeye have been stocked annually since 1989.
Electrofishing Procedures
Electrofishing samples were collected on two dates on each lake during March, April, May, October, and
November, 1996. Samples were collected at fixed sites, with two-person crews (one dipper/one boat driver) using
pulsed DC current (60 pulses/sec; 8-10 amps). Electrofishing sites were stratified by habitat; 1) points and main-lake
shoreline, hereafter referred to as "saugeye" habitat; and 2) coves, hereafter referred to as "bass" habitat. Six 15-
minute units of effort were collected in each habitat type during daylight on each date. Six units of effort also were
collected after dark in "saugeye" habitat on each date for a total of 18 units of effort. The day and night samples in
"saugeye" habitat were collected at the same sites. Data from both sampling dates for each month were pooled (36 units
of effort per lake per month).
Saugeye were the only fish collected during sampling. All fish were measured to the nearest mm and weighed
to the nearest g and with one of four size categories used for analysis; 1) 457 mm (statewide
minimum length limit; hereafter referred to as "large"); and 4) all size classes combined. Spring, 1996 and fall, 1996
samples collected different year classes, particularly in the "small" and "intermediate" size classes. However, stocking
rates were the same for all lakes (50/ha) and assuming relatively similar survival of stocked fish between years, I felt
that this would not compromise the study objectives.
Catch rates (CPUE) were expressed as number of fish per hour of electrofishing. Electrofishing catch rates
for each lake were compared by month and season, day and night, and habitat type for each size group. Catch data
from all lakes were not pooled due to differences in population abundance. CPUE data were log-transformed
[loge(CPUE-f 1)1 to normalize data and stabilize variances. Standard t-tests were used to determine differences in CPUE
by season and habitat type for each lake and size class. Paired t-tests were used to test for differences between day and
night samples collected from "saugeye" habitat. Ryan's multiple comparison test was used to determine differences in
CPUE by month. Statistical significance was assessed at P=0.05.
Sampling precision was measured by determining the coefficient of variation of the mean (CVx=S.E.x1). A
target level of precision was set at CV*=0.125. This value corresponds to the x+0.25x and coincides with standards
established for "management studies " by Robson and Regier (1964). Rearranging the above equation, inserting the
desired level of precision, and solving for N (number of samples) yields the equation:
N=0.125-2x-2s2. (equation 1)
Standard equations for estimating sampling size assume that the data are normally distributed and that the sample mean
and variance are uncorrelated. A mean-variance relationship for this study was calculated from all sampling strata, lakes,
and dates combined by linear regression of loges2 on loge*, yielding the equation:
loges2=1.61+1.221ogex. (equation 2)
The regression equation relating s2 and x was back-transformed to a linear scale and corrected for transformation bias
by adding the mean square error of the regression (MSE)/2. The mean-variance relationship for all samples collected
then becomes:
s2 =exp[(MSE/2) + 1.61 + 1.22x)] (equation 3)
=exp[(0.45/2)+1.61 + 1.22x], (equation 4)
=6.26x '22. (equation 5)
These results were substituted into equation 1 and used to compute sample size requirements:
N=6.26x122x20.125-2 (equation 6)
N=6.26x0780.125-2. (equation 7)
V. Results
Differences in seasonal catch rates were not consistent among lakes nor size classes (Table 1). CPUE of
"small" saugeye was higher in the fall from Holdenville, whereas no seasonal differences in CPUE of the "small" size
class were observed in the Jean Neustadt and Thunderbird data (Table 1). CPUE of the "intermediate" size class was
higher in the fall from Jean Neustadt and Thunderbird; no seasonal difference was detected from Holdenville (Table
1). CPUE of "large" saugeye was higher in the spring from Holdenville and Jean Neustadt; however, higher CPUE's
were found in the fall samples from Thunderbird (Table 1). Catch rates for all size classes combined did not differ by
season from Holdenville and Jean Neustadt, but were higher at Thunderbird in the fall (Table 1).
Analysis of the monthly electrofishing data did not provide additional insight into temporal differences in
CPUE. Therefore, monthly comparisons will not be discussed further.
Precision of most samples was poor. Precision of the Thunderbird data was higher than that for Holdenville
and Jean Neustadt (Table 1). However, a minimum of 5 hours of electrofishing was required to obtain a CVx=0.125
in fall sampling with size classes, day time, and habitats combined (Table 1). Stratifying the data by size class generally
made sampling requirements unrealistic. Ten hours of electrofishing would meet the specified target of precision in
only three of the 24 sampling strata depicted in Table 1.
Diurnal differences in CPUE were more consistent. CPUE's of night samples were higher for all lakes for
the "small" and "intermediate" size classes (Table 2). No diurnal differences in catch rates of "large" saugeye were
found in either season at Holdenville nor in the spring from Thunderbird (Table 2). CPUE of the "large" size class was
higher at night in the spring and during the day in the fall from Jean Neustadt (Table 2). Fall CPUE of the large"
size class was higher during the day at Thunderbird. Catch rates were higher at night for size classes combined tor all
lakes and seasons with the exception of the fall samples from Thunderbird which did not exhibit any diurnal differences
in catch rates (Table 2).
Precision of the night electrofishing samples was higher than the paired samples collected during the day (Table
2). The exceptions to this relationship were at Thunderbird for "large" saugeye and for all size classes combined in
the fall (Table 2). Ten hours of electrofishing (40 units of effort) at night would be required to ensure obtaining a
mean with a 75 % confidence interval based on data from the three lakes in this study (Table 2). By stratifying the data
by day and night, precision was not sacrificed in many cases when the data were broken out by size classes. Eleven
hours of electrofishing would meet the target of precision in 19 of the 48 sampling strata in Table 2. If only night
samples were considered, 15 of 24 strata met the target of precision with 11 hours of electrofishing.
Habitat type had little effect on day time electrofishing sampling efficiency. CPUE was higher in "saugeye"
habitat for the "intermediate" size class in spring at Holdenville and in the fall at Jean Neustadt (Table 3). CPUE of
"large" saugeye was higher in the fall at Thunderbird (Table 3). Catch rates for all size classes combined in "saugeye"
habitat were higher in the spring at Holdenville and in the fall at Jean Neustadt and Thunderbird (Table 3). All other
comparisons of habitat type by size class for each season and lake were nonsignificant. Precision of the data collected
in "saugeye" habitat was higher than the respective data collected in "bass" habitat in 17 of the 24 habitat comparisons
in Table 3. However, stratifying the data by habitat type did little to reach realistic sample size requirements.
VI. Discussion:
This study provided no clear evidence indicating that efficiency would be enhanced by limiting data collection
to a single season. Seasonal differences in catch rates and precision were inconsistent among lakes and size classes.
This is in contrast to the findings of Johnson et al. (1988) who reported higher catch rates of age-1 and older saugeye
from night electrofishing samples in spring than in fall from Pleasant Hill Reservoir, Ohio. Stratifying sampling by
habitat type during daytime electrofishing also did little to improve efficiency.
7
Sampling at night clearly improved efficiency, particularly for the "small" and "intermediate" size classes.
This is consistent with sampling recommendations for collecting age-0 and yearling walleye (McWilliams and Larscheid
1992; Serns 1982). No clear evidence was found indicating that night sampling efficiency for "small" and
"intermediate" saugeye could be improved by stratifying the sampling by season. Differences in CPUE were
inconsistent between spring and fall samples; however, precision of the fall night electrofishing samples was typically
higher than the respective samples collected in spring. In addition, stocking criteria used by ODWC's management staff
(Gilliland and Boxrucker 1995) require data be provided from previous year's stocking to receive subsequent stockings.
Therefore, fall collections of age-0 saugeye fit better into existing management protocol.
Improved capture efficiency was not evident for the "large" size class at night. No differences in CPUE were
observed in the seasonal day-night comparisons from Holdenville. However, catch rates of "large" saugeye from
Holdenville were low throughout the study indicating low population density . This may make drawing any conclusions
relative to capture efficiency of "large" individuals from the Holdenville data suspect. CPUE of "large" saugeye was
higher at night for spring samples and higher during the day in the fall from Jean Neustadt. No diel differences were
seen in the spring data from Thunderbird; however, day samples in the fall had a higher CPUE. Precise data on
"large" saugeye are needed to enhance ODWC's crappie management efforts. Saugeye are used as a tool to reduce
density of overcrowded crappie populations (Gilliland and Boxrucker 1995). It would be useful to develop correlations
between CPUE of "large" saugeye and improvements in crappie size structure and/or growth rates. However, no single
sampling strategy improving capture efficiency was evident from this study.
ODWC's management staff typically expends six units of effort or less per lake to collect data evaluating
saugeye stocking success (catch/h of age-0 saugeye in fall night electrofishing). This amount of effort appears
insufficient to provide the precision needed to meet standards suggested in the literature. Effort needs to be increased
7-fold to provide estimates +.25% of the mean. However, given current time and personnel restraints, an increase in
sampling effort of this magnitude may not be practical. Given a catch rate of 30/h, a resonable estimate for saugeye
457
S.E.
0.24
0.24
mm
N
589
1678
CPUE
9.74a
14.11a
All Sizes
S.E.
1.11
2.61
N
76
55
JEAN NEUSTADT
Spring
Fall
6.93a
7.56a
1.73
1.51
104
96
7.22a
9.89b
1.75
1.73
100
75
5.74a
1.11b
0.83
0.47
124
805
14.04a
11.28a
2.00
1.72
55
67
THUNDERBIRD
Spring
Fall
11.77a
8.56a
1.83
1.84
64
85
16.04a
23.61b
2.26
2.91
49
35
8.94a
14.44b
1.02
1.94
82
54
31.77a
45.72b
3.09
3.93
28
21
11
Table 2. Catch per hour (CPUE) and standard error (S.E.) of saugeye by electrofishing by season and size class from day and night samples collected from
Holdenville, Jean Neustadt, and Thunderbird Reservoirs, 1996. N=number of samples to obtain CV*=0.125. CPUE's within same column with same letter are
not significantly different (paired t-test; P<0.05). Differences across size classes, lakes, and seasons were not tested.
HOLDENVILLE
Season
Spring
Fall
Daytime
Day
Night
Day
Night
CPUE
0.44a
5.67b
0.00a
31.67b
<310mm
S.E.
0.27
1.27
0.00
4.71
N
3171
126
28
CPUE
5.00a
13.89b
0.50a
36.17b
.<400 mm
S.E.
1.15
1.73
0.28
4.70
N
142
56
2624
25
CPUE
1.44a
1.67a
0.50a
0.23a
>457 mm
S.E.
0.43
0.37
0.28
0.54
N
570
476
2624
1206
CPUE
8.33a
18.56b
1.50a
39.17b
All Sizes
S.E.
0.63
4.64
1.72
2.00
N
87
43
543
23
JEAN NEUSTADT
Spring
Fall
Day
Night
Day
Night
0.89a
19.33b
3.00a
19.17b
0.39
4.54
1.00
3.32
1099
42
241
42
1.00a
19.89b
6.67a
21.67b
0.40
4.56
1.88
3.74
931
41
108
38
5.89a
8.56b
2.50a
0.00b
1.35
1.83
1.20
0.00
121
85
294
7.78a
30.67b
9.17a
21.83b
1.58
4.60
2.12
3.71
93
29
80
38
THUNDERBIRD
Spring
Fall
Day
Night
Day
Night
8.33a
18.82b
3.50a
17.83b
3.08
2.79
1.39
4.67
87
43
204
45
12.22a
24.24b
18.67a
39.83b
3..72
3.53
3.90
5.72
62
35
43
23
8.89a
7.88a
27.50a
8.67b
1.40
1.76
4.56
1.01
82
92
31
84
27.67a
38.94b
58.50a
54.50a
4.39
5.21
7.79
5.75
31
24
17
18
12
Table 3. Catch per hour (CPUE) and standard error (S.E.) of saugeye by day time electrofishing by habitat type (Bass-cove habitat; Saugeye-points and main lake
shoreline habitat) and size class from samples collected from Holdenville, Jean Neustadt, and Thunderbird Reservoirs, 1996. N=number of samples to obtain
CVx=0.125. CPUE's within same column for each season with same letter are not significantly different (t-test; P<0.05). Differences across size classes, lakes,
and seasons were not tested.
Season
Spring
Fall
Habitat
Bass
Saugeye
Bass
Saugeye
CPUE
0.11a
0.44a
0.33a
0.00a
<310mm
S.E.
0.11
0.27
0.23
0.00
N
36867
3171
5114
CPUE
0.78a
5.00b
0.33a
0.50a
,<400 mm
S.E.
0.35
1.15
0.23
0.28
HOLDENVILLE
N
1333
142
5114
2624
CPUE
1.11a
1.44a
0.67a
0.50a
>457 mm
S.E.
0.43
0.37
0.28
0.54
N
805
570
1678
2624
CPUE
2.33a
8.33b
1.67a
1.50a
All Sizes
S.E.
0.74
1.72
0.59
0.63
N
319
87
476
543
JEAN NEUSTADT
Spring
Fall
Bass
Saugeye
Bass
Saugeye
0.56a
0.89a
0.50a
3.00b
0.32
0.39
0.37
1.00
2221
1099
2624
241
0.78a
1.00a
1.33a
6.67b
0.38
0.40
0.62
1.88
1333
931
632
108
2.78a
5.89a
0.83a
2.50a
0.83
1.35
0.68
1.20
262
121
1206
294
3.67a
7.78a
2.83a
9.17b
0.96
1.58
1.04
2.12
194
93
256
80
THUNDERBIRD
Spring
Fall
Bass
Saugeye
Bass
Saugeye
8.56a
8.33a
4.33a
3.53a
3.35
3.08
1.38
1.39
85
87
164
204
12.11a
12.22a
12.33a
18.67a
4.17
3.75
3.56
3.90
63
62
62
43
10.00a
8.89a
7.17a
27.50b
2.11
1.40
1.40
4.56
74
82
101
31
29.11a
27.67a
24.17a
58.50b
6.25
4.39
4.33
7.79
30
31
35
17